1. Field of the Invention
This invention relates to the shape of diaphragms to limit stresses when displacing liquids in single or multiple diaphragm pumps and actuators.
2. Background of the Invention
Diaphragms are generally single ply sheets or membranes 6a (FIG. 3) made from metal or plastic, cut into a circular or rectangular pattern. The innermost surface region of the diaphragm is either contiguous across the diameter, but more often is open to create an inside diameter edge 6b. The innermost and outermost edges 6c are welded, bonded, or mechanically fastened in a leak-tight fashion to the pump or actuator elements which produce (pump) or absorb (actuator) displacement of the inner or outer edges of the diaphragm. The relative motion between the inner and outer edges generates fluid pressure and/or flow in pump applications, or derives mechanical force and/or motion in actuator applications.
Pumps and actuators consisting of a single diaphragm are limited in displacement, and therefore, in overall performance. One method to increase the stroking capacity of the pump or actuator is to use a series of diaphragms joined at the inside 5a (FIG. 2) or outside 5 diameter edges. The edges may be welded, bonded, or mechanically fastened. To clarify terminology, a pair of diaphragms joined at either the inside or outside diameter edges is called a convolution. A series of convolutions each joined at the remaining free edge are called a bellows 2 (FIG. 1), or bellows capsule.
In pump and actuator applications, the objective is normally to compress the bellows when mechanical force or pressure is applied. Therefore, for pumps, a mechanical driver element 28 (FIG. 17) located outside the bellows compresses the bellows and squeezes the driven fluid 29 contained within the bellows cavity. In actuator applications, the bellows is encapsulated by a leak tight housing or pressure vessel 34 (FIG. 18). The driving fluid 31 is normally contained between the bellows and vessel, such that when pressure is applied to the fluid, the bellows compresses to move a driven element 32 located inside the bellows.
The proposed art is specific design features of a diaphragm to limit diaphragm stresses in single or multiple diaphragm, or bellows, applications. For subject matter related to the design of individual diaphragms in either a single or multiple diaphragm application, a diaphragm will be subsequently referenced. The end application for the diaphragm design, however, may be a single diaphragm pump, multiple diaphragm (bellows) pump, a single diaphragm actuator, or a multiple diaphragm (bellows) actuator.
The novelty of the proposed art is equally advantageous for pump and actuator applications of single or multiple diaphragm construction. Please note however, that in pump applications, the fluid is contained within the bellows as stated above, and to be advantageous, the diaphragm contour and offset proposed in this specification is located at the innermost radial region 30 (FIG. 17) of the diaphragm. In actuator applications, the fluid is contained outside the bellows, and the diaphragm contour and offset proposed in this specification is located at the outermost radial region 33 (FIG. 18) of the diaphragm. For the purposes of simplifying the specification, a bellows pump will be subsequently referred to as the target application for the proposed art, and as such, the novel design features will be located at the innermost radial region 30. However, the effort to simplify the discussion does not preclude application to an actuator, where the proposed design features are located at the outermost radial region 33 to gain equally advantageous benefits.
Bellows pumps are four stroke reciprocating machines that convey fluids by elongating and contracting the bellows capsule 2 (FIG. 1), therefore displacing fluid volume within the bellows cavity and generating fluid flow and pressure. Such pumps or particularly useful where the conveyed fluid cannot be contaminated by lubricating oil and wear particles, or where the fluid cannot leak across sliding piston seals. Recently, bellows pumps have been employed in industries such as semiconductor process fluid distribution and cryogenics, where fluids must remain very pure, or where leakage of toxic, corrosive agents is not acceptable.
Pump bellows may be categorized as either corrugated one piece construction, where the corrugations are created in a molding or forming process, or by edge welded metal diaphragms, where corrugated diaphragms 4 (FIG. 2) are typically joined by a weld bead 5 at the inside diameter to create the convolution, or pair of diaphragms, as discussed previously. The convolutions are then joined at the outside diameters to create a pleated, expandable capsule. Corrugated tube bellows may be constructed of plastic or metal, and are easier to manufacture than their welded counterparts, but typically do not possess higher performance characteristics such as flexibility and pressure capacity of edge welded designs, and therefore, are usually employed in lower performance applications where fluid pressure is used to actuate the bellows, or in crankshaft driven applications where low flow and compression ratios are permissible.
Higher performance crankshaft driven bellows pumps typically use corrugated edge welded diaphragms due to the greater latitude to create a shape tolerant to high levels of cyclical flexure and pressure. Of primary concern are the flexure and pressure stresses generated by compression of the bellows capsule during the reciprocation process. As the capsule compresses, the diaphragms deflect, resulting in increased displacement induced flexural stresses at the inside and outside diameters of the diaphragms. Fluid pressure also increases as the bellows internal volume reduces, given by the poly-isentropic relationship between fluid pressure and volume, where an ideal gas is the selected fluid.P1/P2=(V2/V1)k  (Equation 1)Where,P=Fluid pressure at two distinct points of the reciprocation process,V=Bellows internal volume at the distinct reciprocation points, andk=Isentropic or poly-isentropic coefficient of the fluid conveyed.
For and incompressible fluid, other relationships related to the viscosity of the fluid and the change in volume over time, or volumetric flow rate, would apply.P=f(ν,Q)Where,ν=Fluid Viscosity, andQ=Volumetric Flow Rate (ΔV/Δt).
The fluid pressure also generates stresses throughout the diaphragm, which typically add to deflection stresses at the inside diameter, and subtract from deflection stresses at the outside diameter. The stress combination particularly at the inside diameter can significantly limit the pressure range of the bellows, as the cyclic nature of the combined deflection and pressure stresses may produce considerable fatigue damage.
The prior art of crankshaft driven edge welded bellows pumps is generally limited to pressure below 100 psi, due to the high stresses on the unsupported diaphragm spans resulting from both fluid pressure and deflections exercised during the fluid compression process, particularly at the inner diameter location of the diaphragm as discussed above. Where higher pressures approaching 100 psi are attainable, diaphragm material selection is limited to highly fatigue resistant materials such as non-heat treated AM-350 stainless steel to mitigate cyclic fatigue damage. However, such materials are limited in corrosion resistance, and are not suitable for certain corrosive process applications. Additionally, pressures in excess of 100 psi typically require that the bellows or diaphragm be externally air or fluid driven 6 (FIG. 3) to counteract the high pressures internal to the diaphragm. However, such designs are more costly, pump at slower rates, may require a second stage, and therefore would require a supplemental system 7 to regulate and control pressure on the secondary fluid driving side of the diaphragms.
In addition to structural problems under relatively high pressures, traditional edge welded bellows pumps are typically unable to achieve high pressures without the use of two stages 8 (FIG. 4) coupled with a crossover pipe 9. Most bellows require substantial spacing between diaphragms to prevent cyclical galling from inadvertent diaphragm contact, and from a basic practical perspective, for weld bead clearance. As a result, a substantial uncompressed dead volume may drastically reduce overall compression ratio, resulting in poor efficiency and fluid output. Consequently, pumps typically require multiple stages at increased product cost to achieve overall desired system compression ratios.
3. Objects and Advantages
The objects and advantages of the proposed invention are:                a) An engineered diaphragm shape that contacts adjacent diaphragms at the onset of a predefined stress limit, thereby reducing the unsupported span of the diaphragms, and preventing further increases in diaphragm stress in the fully contacted portions of the diaphragm span.        b) A unique diaphragm shape having the ability to inherently control stresses as said in paragraph “a” above throughout all intervals of the compression cycle, thereby permitting a wider range of materials to be considered for fatigue and other non-fatigue design requirements, such as, but not limited to, corrosion resistance.        c) A unique diaphragm shape as described in paragraphs “a” and “b” above that will permit higher pressures to be employed due to the progressive stress mitigating nesting under deflection and associated pressure.        d) Lubricious wear strips that prevent galling and premature wear of the diaphragms under the cyclic contact of paragraph “c”.        e) A wear strip offset incorporated in to the diaphragm shape that places the wear strip between the diaphragms. The offset also permits higher compression ratios in the presence of enlarged weld beads, as a result of the minimal unstroked fluid dead volume inherent in the design, ultimately achieving higher pump performance at significantly less cost.        
Further objects and advantages of my design will become apparent from a consideration of the drawings and ensuing description.